A side airbag apparatus has an airbag assembly that protects a passenger seated on a seat in a passenger compartment by being inflated and deployed at a side wall portion of the passenger compartment corresponding to the seat in a collision of a vehicle. The airbag assembly includes a main airbag and an auxiliary airbag that inflate and deployed in a state arranged along a lateral direction of the vehicle. If the body size of the passenger is greater than or equal to a predetermined size, the main airbag is inflated and deployed and the auxiliary airbag is not inflated or deployed. If the body size of the passenger is smaller than the predetermined size, the main airbag and the auxiliary airbag are both inflated and deployed. Thus, in the latter case, the airbag assembly in the inflated and deployed state has a great thickness in the lateral direction of the vehicle compared to the case in which the body size of the passenger is greater than or equal to the predetermined size.